As a device size has been reduced to a deep sub-micrometer, gate control capability of a conventional planar transistor is getting down, and a short channel effect is becoming more and more obvious, thus leading to a series of problems, such as drift of threshold voltage, increase of sub-threshold slope and leakage current in sub-threshold region, drain induced barrier lowering effect and the like. In order to suppress the short channel effect, a multi-gate structure can be used to enhance the control capability for the gate to the channel. An ultimate of the multi-gate structure is a surrounding-gate silicon nanowire structure. Due to an excellent gate-control capability and a one-dimensional quasi-ballistic transport of the surrounding-gate structure, the surrounding-gate silicon nanowire transistor can have an excellent sub-threshold characteristic, an increased current on-off ratio, an enhanced current drivability. Further, it is also well compatible with a CMOS process, and therefore, the surrounding-gate silicon nanowire transistor has been considered to be a promising device that will replace the planar transistor in the future.
However, as shown in FIG. 1, in the surrounding-gate silicon nanowire transistor, a fringing capacitance between the gate and the source/drain may be an unnegligible issue because of the structural characteristics of the surrounding gate and the nanometer-sized channel, thus adversely affecting the transient response characteristics of the device.
In order to reduce the parasitic capacitance, a material with a lower dielectric constant can be used as sidewalls to reduce a capacitive coupling effect between the gate and the source/drain, so that the fringing capacitance can be reduced. To this end, air is one of the materials having a very low dielectric constant. FIG. 2 illustrates a cross-sectional view along a channel direction of the surrounding-gate silicon nanowire transistor with air sidewalls.
Due to a peculiar three dimensional structure of the nanowire, it is required certain special processes to form the air sidewalls, which has not been reported so far in this research area.